The present invention relates to the use of a peptide from the circumsporozoite (CS) protein of Plasmodium falciparum (P. falciparum) and the derivatives thereof as a universally recognized T-cell epitope, i.e. an epitope which is recognized in association with many different human and mouse major histocompatibility complex (MHC) haplotypes, e.g. in the context of the human MHC class II molecules such as DR1, DR2, DR4, DR5, DRw6, DR7 or DR9. Furthermore the present invention relates to the above-mentioned peptides per se and to immunogenic compositions comprising such a peptide or a derivative thereof. These immunogenic compositions can be used as vaccines to elicit a durable immune response against a pathogenic agent in humans and animals irrespective of the MHC haplotype of the host.
It is known that chemically synthesized peptides representing selected regions of antigenic structures (B-cell epitopes) can induce antibodies which bind to the native molecules (Arnon et al., Proc. Natl. Acad. Sci. USA 68, 1450-1455 [1971]). Such peptides may be injected into a host whereby a protective antibody response is induced (for a review see Shinnick et al., Ann. Rev. Microbiol. 37, 425-446 [1983]).
However the strict genetic control of responsiveness to individual epitopes by the polymorphic class II MHC genes limits the usefulness of single epitope vaccines.
An example of an epitope which does not always elicit an immune response in a host is the repeated sequence Asn-Ala-Asn-Pro (NANP) in the CS protein of the malaria parasite P. falciparum (Enea et al., Science, 225, 628-630 [1984]; Dame et al., Science 225, 593-599 [1984]). The repetitive peptide was found to induce a parasite-specific immune response only in those mice carrying the H-2.sup.b haplotype. (Good et al., J. Exp. Med. 164, 655-660 [1986]; del Guidice et al., J. Immunol. 137, 2952-2955 [1986]).
Recently it has been shown that the non-immunogenic B-cell epitope of the CS protein (NANP).sub.n can be made strongly immunogenic by conjugation to a T-cell epitope comprising amino acid residues 326 to 343 from the CS protein (Good et al., Science, 235, 1059-1062 [1987]). A peptide comprising an amino acid sequence corresponding to this T-cell epitope was covalently linked to a peptide comprising the repeat sequence (NANP).sub.5. The combined peptides elicited high titers of antibodies in BlOBR and BlO.A(4R) mice. Similarly Francis et al. have reported in Nature 330, 168-170 [1987]that non-responsiveness to a foot-and-mouth disease virus peptide may be overcome by combining the foot and mouse disease virus B-cell epitope with foreign helper T-cell determinants, e.g. from ovalbumin or sperm-whale myoglobin. Responses to the T-cell determinants described by Good et al. ([1987], supra) and Francis et al. (supra) were under the control of Ir genes (immune response genes). This means that only specific inbred mouse strains having the "right" MHC haplotype could recognize the T-cell epitopes used.
Since an ideal vaccine has to elicit an immune response against a pathogenic agent in all individuals it has to include T-cell epitope(s) which are recognized by all MHC haplotypes.
It has now been found that the CS.T3 peptide having the amino acid sequence ##STR1## can be used as a universally recognized T-cell epitope. This means it is recognized in association with many different human and mouse MHC haplotypes e.g. in the context of the human MHC molecules DR1, DR2, DR4, DR5, DRw6, DR7 or DR9. The CS.T3 peptide corresponds to the residues 378 to 398 of the CS protein from P. falciparum (Dame et al., supra), but contains two alanine residues in place of the native protein's cysteine residues at Position 384 and 389. The CS.T3 peptide can therefore also be called [Ala.sup.384,389 ]P.falciparum CS(378-398).
Furthermore it has been found that derivatives of the CS.T3 peptide having minor modifications in the amino acid sequence of the peptide CS.T3 may still be used as universally recognized T-cell epitopes. Thus for example one or two amino acids may be deleted at either end of the peptide without impairing its use as a universally recognized T-cell epitope. When more than two amino acids are deleted at either end of the CS.T3 peptide the peptide may still be recognized by almost all MHC haplotypes although it has been observed that the more amino acids are deleted the more the peptide loses its capability to be recognized by different MHC haplotypes. When more than about eight amino acids are deleted at either end of the peptide it is no longer recognized as T-cell epitope by any MHC haplotype (see below).
Other modifications in the amino acid sequence of the CS.T3 peptide which may have no effect on its use as a universally recognized T-cell epitope are amino acid substitutions and additions at the C-terminus and/or the N-terminus. Thus the said CS.T3 peptide or the derivatives thereof may be part of a larger polypeptide e.g. the natural CS protein or fragments thereof or a fusion protein containing foreign peptide sequences preferably peptide sequences from another polypeptide of a malaria parasite. Furthermore the C-terminus of the CS.T3 peptide or the derivatives thereof may be amidated.
Besides modifications at the N- or the C-terminus of the peptide, modifications within the amino acid sequence of the CS.T3 peptide or its derivatives may be possible which modifications still enable the peptide or its derivatives to be used as a universally recognized T-cell epitope. These modifications may be deletions, insertions and/or amino acid substitutions. Examples of such derivatives are peptides comprising residues 378 to 398 of the CS-protein having cysteine residues at position 384 and 389 as in the native CS-protein. The general features of the modifications are that they do practically not alter the secondary or tertiary structure of the peptide (Doolittle, R. F., in "The Proteins", Vol. IV, Neurath, H. and Hill R. L., Eds., Academic Press, New York, p. 1-119. [1979]). The derivatives mentioned above must bind to the MHC class II molecules at least as well or preferably better than the CS.T3 peptide. It has been observed that derivatives in which Ile at position 383 was replaced by Leu and/or Glu at position 387 was replaced by Gly bind to both DR5 and DRw6 about 10-100 times better than the original CS.T3 sequence when the binding was measured by a competitive binding assay (Kilgus et al., Proc. Natl. Acad. Sci. U.S.A., 86, 1629-1633 [1989]).